SlowlyMigrating from Cwicly
Move from Cwicly to Slowly by installing the plugin. Your site keeps working.
Slowly is the continuation of Cwicly, so moving across is mostly just installing the plugin. The blocks are the same ones underneath, and the sites you built keep working.
The short version
Install the Slowly plugin and keep the theme you are on. Your pages, templates, and menus stay exactly where they are, because nothing about your theme has changed. There is no export and no rebuild.
If you switch themes
The one thing to know is the same as on the theme and plugin page. WordPress stores templates and menus against the active theme, so if you switch from the Cwicly theme to the Slowly theme, or any other, your templates appear to vanish. They are still filed under the old theme.
They are not lost. The Themer notices the orphaned templates and offers to adopt them, re-filing your templates and parts onto the theme you switched to. Run that once and everything comes back, now homed on the new theme.
If you would rather not deal with any of that, staying on your current theme avoids it entirely.